The invention relates to a projection exposure system having a light source generating a pulsed light according to the type defined in more detail in the preamble of claim 1. The invention also relates to an exposure method in microlithography.
In conjunction with linearly polarized light sources such as, for example, lasers, illuminating devices of projection exposure systems, in particular lithography objectives, have optical polarization properties which no longer meet the current requirements in all cases. Frequently, linearly polarized laser light with a polarization degree of approximately 90-95xc2x0 is rendered circular by means of a lambda/4 plate. The aim is to maintain this circular state up to a wafer which is to be exposed. However, it is disadvantageous that the optical components in the illuminating system, in the objective and reticle structures introduce phase differences. As a result, the original circular laser light acquires an elliptical polarization state. The disadvantage of this is the production of an uncontrolled state which varies from objective and illumination with reference to the respective next pair. This results in nonuniform brightness distributions over the image field and uncontrolled influences on the imaging and the image production.
These errors can presently be reduced only by materials which have little stress, mirror coatings which maintain polarization, dispensing with a glass rod and the like, all of which is possible, however, only at considerable outlay.
U.S. Pat. No. 5,673,103 describes an illuminating system in photolithography, an arrangement for rotating the polarization direction with the aid of a lambda/2 plate being provided. In order to achieve illumination with linearly polarized light of changing direction in conjunction with enhanced resolution or depth of field, the lamda/2 plate is rotated toward a focusing lens and an optical integrator.
It is the object of the present invention to create a projection exposure system and an exposure method in microlithography in which the disadvantages of the prior art are avoided, in particular in which nonuniform brightness distribution over the image field and uncontrolled influences on the imaging and image production do not occur.
According to the invention, this object is achieved by means of the features named in the characterizing part of claim 1.
An exposure method according to the invention is described in claim 11.
Using the system and the method according to the invention, it is possible in practice to leave virtually unchanged the known systems without complicated changes and yet, in the process, necessarily to bring about an optical polarization effect, specifically to expose the wafer in an unpolarized fashion. This purpose is served by the element changing the polarization. This can, for example, be achieved in a simple way by a lambda/2 plate which is arranged between the end of the beam feed of the linearly polarized light and the input of the illuminating device or the illuminating system.
It is known that a lambda/2 plate rotates the polarization direction of a linear light. If, in this case, the lambda/2 plate rotates with its axis, which is situated on the optical axis or parallel thereto, then the emerging linear laser light rotates with its vibration direction at this double frequency. The individual pulse of the light source, for example a litholaser, can now be synchronized with the rotary movement of the lambda/2 plate. In this case, the lambda/2 plate rotates further by 45xc2x0 from pulse to pulse. The laser light entering the illuminating system remains linear and changes in each case by 90xc2x0 from pulse to pulse. If a defined position of the crystal axis relative to the pulse in the lambda/2 plate has been set, that is to say 0xc2x0, 45xc2x0, 90xc2x0, 135xc2x0, 180xc2x0 etc., the polarization can reach approximately 0xc2x0 and 90xc2x0 relative to the mirror edges of the illuminating system. The decisive point in this is: 0xc2x0 and 90xc2x0 polarization between two pulses is now exactly orthogonal. If an exposure is now carried out with the aid of these two orthogonal pulses which integrates the resist of the wafer, this corresponds in sum as regards the result of exposure precisely to that of two completely unpolarized pulses. This means a cancellation of the polarized light or virtually depolarized light. There are always a sufficient number of exposure pairs present owing to the high frequency in the case, for example, of the use of litholasers at approximately 2000 to 8000 Hz.
In practice, the pulses mutually cancel one another out in their polarization effects downstream of the lambda/2 plate, specifically in pairs, or they supplement one another to produce an unpolarized effect.
If a lambda/4 plate, for example, is left in the illuminating system, the result in the presence of phase lags is a pulse pair with left-elliptically polarized light and right-elliptically polarized light, which is equivalent to entirely circular light in exposure, and this is the aim of the existing systems.
Polarization-rotating elements in the downstream beam path thus do not disturb the effect according to the invention. Polarization-selecting elements have their interference at least reduced.
The additional outlay in the system owing to a rotating synchronized lambda/2 plate is negligible by comparison with known devices for avoiding the disadvantages outlined.
A further advantage of the invention consists in that it is also possible for subsequently existing projection exposure systems to be depolarized in this way, the result being to produce new possible applications in a cost effective way.